"Where/who, exactly, are the jihadists? You're suggesting the Muslims in London carry out vigilante justice against people whom they believe are jihadists?"
I don't think anyone is expecting "vigilante justice".
However, I'd bet my house, and yours, that someone in that group knows someone who knows someone who knows who the chicken-$#!^s that did this are!
I would bet that the leaders of what ever mosque they attend knows who they are and endorsed the act.